Cyclic(Alkyl)(Amino)Carbene (CAAC)–Mercury(II) Complexes and their Catalytic Activity in Hydroamination Reactions

Abstract

The unprecedented low valent metal compounds, highly active catalysts are the outcome of the stabilization offered by the appended carbenes to the metal centers.A modified version of NHCs named as cyclic(alkyl)(amino)carbenes (CAACs) discovered by Bertrand in 2005, have surpassed the notion of routine carbene chemistry due to more nucleophilic (σ-donating) as well as electrophilic (π-accepting) nature than those of diamino carbenes.The present work deals with the syntheses of adducts of Cyclic (Alkyl)(Amino)Carbene (CAACs) with HgX2salts. As an initial trial, when Hg(OAc)2was reacted with [CAACH]+[Cl]–the ionic complex, [CAACH]+[HgCl3]–(1)was isolated. In subsequent attempts, the reaction of insitu generated free carbene was carried out with HgX2salts that smoothly yield the CAAC-Hg(II) adducts as[CAAC∙HgCl(μ-Cl)]2(2), [CAAC∙HgBr(μ-Br)]2 (3),[CAAC∙HgI(μ-I)]2 (4). In an effortto substitute the halide with a weakly coordinating anion, a cationic mercury species,[(CAACcy)2Hg(H2O)]2+2[NO3]-(5)was also obtained. This product was isolated by reacting AgNO3 with the previously synthesized complexby Singh and co-workers [CAACcy∙HgBr(μ-Br)]2 (3.1)in DMSO.When the same reaction was performed in dry THF a two coordinated cationic mercury complex,[(CAACcy)2Hg]2+[Hg2Br6]2-(6)was isolated.After the successful synthesis of some of these adducts, their application in the hydroamination reactions between aromatic amines and terminal alkynes has been explored. The catalytic ability of [CAACcy∙HgBr(μ-Br)]2 (3.1)in intermolecular hydroamination has been explored in detail.